The long-term objective of this proposal is to establish the molecular basis of host cell variation. To this end, the specific aims of this project are: (1) To determine which host cell isolate(s) of influenza viruses represent those replicating in the infected human and their pathogenicity; (2) to determine the role of different influenza gene products in host cell variation, and (3) to determine if there are differences in the immune response to host cell variants. Both direct and indirect approaches will be taken to address the first specific aim. A panel of discriminating monoclonal antibodies will be used to examine directly, the antigenic phenotype of the HA of viruses present in an original throat wash sample, using immunological and electron microscopy techniques. The molecular structure of critical regions of the HA molecule will be determined by gene amplification and molecular cloning of the viral genome existing in original samples. The results from these direct approaches will be correlated with the antigenic and structural identity of viruses from the same individual. isolated indirectly by growth in mammalian and egg host cells. The mechanism whereby antigenic and structural variants may arise and circulate in a population will be determined both in humans and in a ferret model system. The molecular changes in the HA between host cell variants will be determined including their amino acid sequence, post-translational modifications, receptor-binding and fusion properties of the virus. ln addition, the role of other viral gene products such as the matrix (M), nucleoprotein (NP), and neuraminidase (NA) proteins will be investigated. The protective efficacy of vaccines prepared from mammalian cell- and egg-grown variants will be established in ferret and mouse animal models. The antibody and cellular immune responses to these vaccines will be correlated with their relative protective effects. The proposed studies will provide fundamental information on influenza virus variation that will establish a scientific basis for selection of host cell-grown influenza virus strains which most closely resemble the virus replicating in humans, for use in epidemiological studies and effective influenza vaccines.